At first sight, BBDO New York's film Evan – for US organisation Sandy Hook Promise – is just another high school love story. But it's one that has a gut punch ending that most viewers won't have seen coming. In the foreground it follows a developing teenage romance, but something else entirely is hidden away in the background – the telltale signs that can precede gun violence.

“There are a lot of organisations talking about the role guns play in gun violence, which is a divisive conversation in the US,” says Creative Director Peter Alsante, who also worked with Sandy Hook Promise on its Wood Pencil-winning What They Left Behind documentary, which explores the tragic shooting of three American children.

“You won't attract people talking about more guns or less guns, so Sandy Hook Promise's focus is to talk about life on either side of the gun,” he adds. “It's about people, and that's one of the biggest problems in the conversation to stop gun violence – we can be so focused on laws and rights that we've forgotten that it's people's lives at the core of this whole issue.”

The organisation wanted to raise awareness of some of the warning signs of gun violence, but also create a deeper connection to the issue – making film an obvious solution.

“People can't relate to it,” says Alsante. “Unless you've experienced gun violence in your life, it feels like such an abstract concept. It can feel like something that happens somewhere else. Being able to connect with people on a personal level and make it more relatable was a big part of our mission to infiltrate people's thought processes about gun violence, and it being preventable.”

The agency worked with director Henry-Alex Rubin, who helped bring their idea of a hidden narrative to life. Using the high school romance genre made sense, as it was an environment and a story that many viewers would be able to relate to, however the challenge came when finding the right balance to give the foreground and background story.

“Quite frequently in advertising, and I'm sure in any creative endeavour, you look at the idea on the piece of paper and think, 'Wow, this could be great… if we don't totally f*ck it up',” says Alsante. “This was even more so because we had to walk such a fine line.”

The process required a serious leap of faith, and the creative director says that the shoot had its fair share of fraught moments when debating just how secret the 'secret' narrative needed to be. To make sure the viewer stayed distracted from the real story being told, the director also incorporated a few visual red herrings – secondary parts of foreground narrative, and distracting details that would catch the viewers' eye.

Rather than releasing Evan as an ad – which the agency felt would have made viewers suspicious – BBDO used its blogger connections to share the film.

“The idea was people clicking play would have no idea what they were about to watch, and they'd be so emotionally invested in this as an unbranded short film that when we pulled the rug, it would be like a punch to the gut,” says Alsante.

The approach worked, with Evan racking up more than 100 million views in less than a week – making it one of the biggest viral successes the creative director has been involved with so far.

However for Alsante the project has been about more than scoring views, it's been a chance to gain a better understanding of the impact gun violence has on everyday people.

“When I started working with Sandy Hook Violence, it was just another work project,” he says. “But over the years I've spent so much time with these parents that have lost children to gun violence, and have seen how our work affects them on a deep emotional level. It's opened my eyes to the power we have.”

Evan's impact has ultimately gone beyond its millions of online viewers, with the film now used as a training tool by the United States of Homeland Security, as well as the Disney corporation and dozens of schools and police organisations around the US.

D&AD Impact seeks to identify and celebrate great, transformative ideas that contribute towards a better, fairer and more sustainable future for all.